The present invention relates generally to gas turbine engine combustors, and more particularly to a combustor including a mixer having multiple injectors.
Fuel and air are mixed and burned in combustors of aircraft engines to heat flowpath gases. The combustors include an outer liner and an inner liner defining an annular combustion chamber in which the fuel and air are mixed and burned. A dome mounted at the upstream end of the combustion chamber includes mixers for mixing fuel and air. Ignitors mounted downstream from the mixers ignite the mixture so it burns in the combustion chamber.
Governmental agencies and industry organizations regulate the emission of nitrogen oxides (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from aircraft. These emissions are formed in the combustors and generally fall into two classes, those formed due to high flame temperatures and those formed due to low flame temperatures. In order to minimize emissions, the reactants must be well mixed so that burning will occur evenly throughout the mixture without hot spots which increase NOx emissions or cold spots which increase CO and HC emissions. Thus, there is a need in the industry for combustors having improved mixing and reduced emissions.
Some prior art combustors such as rich dome combustors 10 as shown in FIG. 1 have mixers 12 which provide a rich fuel-to-air ratio adjacent an upstream end 14 of the combustor. Because additional air is added through dilution holes 16 in the combustor 10, the fuel-to-air ratio is lean at a downstream end 18 of a combustor opposite the upstream end 14. In order to improve engine efficiency and reduce fuel consumption, combustor designers have increased the operating pressure ratio of the gas turbine engines. However, as the operating pressure ratios increase, the combustor temperatures increase. Eventually the temperatures and pressures reach a threshold at which the fuel-air reaction occurs much faster than mixing. This results in local hot spots and increased NOx emissions.
Lean dome combustors 20 as shown in FIG. 2 have the potential to prevent local hot spots. These combustors 20 have two rows of mixers 22, 24 allowing the combustor to be tuned for operation at different conditions. The outer row of mixers 24 is designed to operate efficiently at idle conditions. At higher power settings such as takeoff and cruise, both rows of mixers 22, 24 are used, although the majority of fuel and air are supplied to the inner row of mixers. The inner mixers 22 are designed to operate most efficiently with lower NOx emissions at high power settings. Although the inner and outer mixers 22, 24 are optimally tuned, the regions between the mixers may have cold spots which produce increased HC and CO emissions.
Among the several features of the present invention may be noted the provision of a mixer assembly for use in a combustion chamber of a gas turbine engine. The assembly includes a pilot mixer and a main mixer. The pilot mixer includes an annular pilot housing having a hollow interior, a pilot fuel nozzle mounted in the housing adapted for dispensing droplets of fuel to the hollow interior of the pilot housing, and a plurality of concentrically mounted axial swirlers positioned upstream from the pilot fuel nozzle. Each of the swirlers has a plurality of vanes for swirling air traveling through the respective swirler to mix air and the droplets of fuel dispensed by the pilot fuel nozzle. The main mixer includes a main housing surrounding the pilot housing defining an annular cavity, a plurality of fuel injection ports for introducing fuel into the cavity, and a swirler positioned upstream from the plurality of fuel injection ports having a plurality of vanes for swirling air traveling through the swirler to mix air and the droplets of fuel dispensed by the fuel injection ports.
Other features of the present invention will be in part apparent and in part pointed out hereinafter.